Adding Training to TorchSharp: First Step

[interesaaat]

The new version of TorchSharp with training requires the addition of a C API over libTorch. The C API is contained into an external repository called LibTorchSharp. I chose to have a separate repo because I expect eventually LibTorchSharp to go away because:

• libtorch will directly provide the C API, or
• LibTorchSharp will be merged into libtorch, or
• LibTorchSharp will be automatically generated from PyTorch / libtorch.

Anyway, it will be great if we can move LibTorchSharp from my local repo to a xamarin repo and while we do this:

• we do code review,
• we make sure that it compiles over different platforms (for the moment I only tested it on Windows)
• we add Azure Pipeline to automate compilation and the creation of nuget
• we add the nuget to TorchSharp.

After all these, I think I can submit the PR adding training to TorchSharp.

[interesaaat]

Closing because the discussion is happening in #79.